"anorton" wrote in message
m...

If you convert the shade number (S) into something called "optical density' (OD),
then the OD numbers do add up when you combine them. the formula is:

OD = 0.428 * (S -1)

To convert back from OD to shade # the formula is:

S = 2.33*OD +1

So a shade number of 7 has an OD of 2.568 . Two of these used together have an OD
of 5.136 which is a shade number of 12.96 An OD of 5 or more is usually
considered safe enough for sun viewing.

I have no idea why the welders shade scale was made different than optical density.

Assuming your equations for Shade # vs OD are correct. Then, since
the equations are linear, you can add Shade #'s too by using
S(sum) = S1 + S2 -1.
Art